Page:Popular Science Monthly Volume 90.djvu/493

 Popular Science Monthly

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��brass rod with 6 hexagon brass nuts to fit, a number of perfectly flat brass or copper washers of about ^ in. outside diameter, which will slip over the 5/32-in. bolt, a little soft brass strip, two small screws and a hand wheel. In Fig. 6 is shown how one of the 8-32 bolts is used to support the top of each corner of the group of fixed plates in proper relation to the top of the con- denser itself. As indicated, the completed assembly begins at the top with a hexagon nut and washer, after which comes the 5-in. square top plate. This is clamped in place by means of a second washer and nut, after which come separating washers to space the uppermost fixed plate the proper distance from the lower side of the insulat- ing top, and then that plate itself. After the first plate, enough washers to make a space of 3-^ in. are put on the threaded rod, and then the next plate. Thus the thirteen fixed plates are placed on and clamped by the nuts of four of the threaded rods. At the bottom, as shown in Fig. 7, the third nut is screwed upward to hold the plates firmly, and a fourth nut turned on to the point which will hold the lowest fixed plate the correct distance from the upper side of the insulating base. After adding a washer, the base itself is placed on, and then a threaded rubber, wood or fiber foot is screwed on to clamp it all in place. Of course, only the corner posts of the base itself take these feet; the two rods at the inner edges of the fixed plates are fastened by screws and washers on the lower side of the base.

The two corners of the base, shown at the lower part of Fig. 5, where no fixed plates reach, are supported by the fifth and sixth threaded 8-32-rods in the manner shown in Fig. 6 and 7, with the obvious exception that the plates themselves and their separating washers are omitted.

The Rotary Part

The moving plates are assembled upon the 5/32-in. threaded rod, clamped by the hexagon nuts and separated by the larger washers, as shown in Fig. 8 and 9. Begin- ning at the top of Fig. 8, it is seen that the hand-wheel, which may be a knurled disk of J<^-in. hard rubber 2 in. in diameter, is clamped at the top of the 5/32-in. rod or shaft between two washers and nuts. Be- tween the lower nut and washer there is gripped the end of a bent metal indicator- hand which points to a degree scale (a cheap protractor makes a good one)

��mounted on the upper side of the insulating top. Just below this lower nut is a little brass bushing or tube, forced into the in- sulating top and having the right inside diameter to act as a bearing for the shaft. Below this come separating washers— one or more — and then two nuts which lock each other and hold the moving plates in place. The uppermost moving or semi-circular plate comes next; immediately below it, and separating it from the next plate, are enough of the larger washers to space the plates just 14: i"- apart.

As shown in Fig. 9, the moving section is built downward, and the lowest plate is clamped in place by two more nuts. The shaft itself continues for about yi in. farther, and is tapered off to a blunt point so as tc reduce friction.

The Lower Bearing

In Pig. 9 is also shown how two pieces of I 'i6-in. soft brass strip are bent and

���The machine screw fastening at the upper and lower corners and the main shaft bearings

drilled to form a thrust bearing for the lower end of the shaft. They are secured to the base or lower 5-in. square insulating plate by the two machine screws. Elec- trical connection is made to the moving plates by soldering a wire to one of these screws, running it out to one of the corner screws which is not associated with the fixed plates, and once more soldering it there. A binding post takes the place of the top nut of Fig. 6 on this outside screw. Similarly, a second binding post substi- tuted for the uppermost nut on one of the screws supporting the fixed plates provides a convenient means of connection with the fixed plates.

For continued service it will not do to

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